how can i make my apple gift card be my main payment option

Once you redeem your Gift Card, the money will go into your Apple Account balance. Most purchases you make will come out of the account balance first before being charged to another payment method.

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

Apple Account will always be used first if funds are available in the account. Apple Cash (if enabled) is first payment method if no funds are in Apple Account. Credit and or debit cards are next and their order of use can be changed with cards, but cannot be used before Apple Cash or Apple Account.

Even with an Account balance you may need another valid payment method. The purchases will be deducted from your Account balance before charging your other payment method. I assume you are getting the Update Payment Information message when attempting to download apps. Other users who have received this message with an Account Balance have not followed up with the resolution after contacting Apple. Apple provides no details on why this may necessary, but some possible reasons could be:

• You owe more than the $25 on your account.
• Verify the region set on your device. It must match the payment method. Each region has their own App Store and some users have changed the region on their device in an attempt to use Apps/Subscriptions that may not be available to them in their own region.

The only way to know for sure is to contact Apple directly, as they are the only ones that have access to your account information and resolve the issue. If you could follow up with the solution provided for you, it may help other users.

Contact Apple for support and service - Apple Support

Please follow the steps in this Apple Support article on how to redeem your Apple Gift Card.

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

Add a payment method to your Apple ID - Apple Support